1. Field of the Invention
The present invention is related to output signal drivers, more specifically to differential signal drivers that have a wave shaping, i.e. emphasis, capability, and further more specifically to a differential output driver suitable for current mode logic, CML, applications.
2. Description of the Related Art
With reference to FIG. 1, a series of binary logic signals transmitted to a receiver buffer 11 may consist of a series of high and low pulses representing logic high and logic low signals, respectively. This typical type of signal transmission uses a single signal line to transmit logic 1's and 0's (i.e. logic high and low signals) by means of voltage high (such as VCC) and voltage low (such as GND) levels. Although simple to implement, this type of signal transmission deteriorates as transmission frequencies and/or the communication lines are increased.
To improve transmission integrity and signal recovery, differential signal transmissions can be used, as depicted in FIG. 2. In this case, two lines are used to transmit a logic signal. A first line transmits the true logic form 13 of a logic signal and a second line transmits the complementary logic form 15 of the signal. Both lines are applied to the input of a differential amplifier 17 at the receiver end of the transmission. Since the differential amplifier 17 can discern a received logic signal by comparing the relative voltages of the two signal lines and determining which of the two lines is at a higher potential, it is not necessary for either line to maintain a fully logic high voltage level (VCC) or a fully logic low voltage level (GND) all the way along the transmission line to receiver 17. Therefore, differential communication systems can achieve higher frequencies and longer communication lines than can single-end communication systems. As frequencies are increases even further, however, this implementation of a typical differential transmission system also begins to experience signal deterioration and signal recovery issues.
High frequency designs, i.e. in the gigahertz range, are challenging because of second order effects in the physical transmission medium, i.e. the transmission lines themselves. These second order effects can be neglected at lower frequencies but are dominant at higher frequencies. Transmission line skin effect, dielectric loss and discontinuities due to geometry changes in the signal lines all contribute to signal degradation, i.e. to altering the shape of the traveling wave. One way of mitigating these degenerative affects of the physical medium is to shape the driven signal pulses (from an output driver) in such a way as to produce a better signal to noise ratio at the receiving end.
It has been found that the success rate of signal recovery can be increased by implementing a technique known as emphasis, or pre-emphasis, (i.e. a wave shaping technique implemented at the output driver side of a driver-receiver communication pair for better signal recovery at the receiver side). Although emphasis techniques can be applied to single-ended transmission systems, it is most often associated with differential signal transmission systems.
Multiple emphasis application techniques are known, but a common emphasis technique improves signal recovery by increasing the voltage (and/or current) level of a transmitted logic signal at logic transitions. For example in FIG. 3, a transmitted true logic signal 21 and its complementary logic signal 23 are given an increased in voltage magnitude at logic transitions (i.e. when transitioning from a logic “1” to a logic “0”, and vise-versa, as exemplarily shown following the logic transitions of FIG. 1 from right to left). These logic transitions are identified by label “Tr” in FIG. 3. If no logic transition occurs in successive signal transmissions, emphasis shaping is removed (i.e. the voltage swing levels return to non-emphasis levels) until the next logic transition.
In FIG. 3, the right-side of the pulse train represented earlier transmitted pulses traveling toward differential amplifier 19, and the left side of the pulse train represent more recently transmitted signals placed on the communication line by a signal transmitter, not shown. Thus, looking at the pulse train from right to left (i.e. from earlier transmissions to more current transmissions), one can identify pulses where logic transitions took place, as identified by label Tr. For example, the last four logic signal pulses shown at the left-side (i.e. the transmitter side) of the pulse train are “1 0 1 1”, and thus experienced no logic transition between the earliest-two consecutive 1's, but did experience a logic transition at the most recent two pulses, “1 0”. The magnitudes of the voltage high and voltage low levels of the logic transitions are therefore increased, i.e. emphasized, or pre-emphasized. However, when no logic transitions occur in consecutive signal pulses, such as those pulses not identified by the label “Tr”, the magnitudes of the voltage high and voltage low levels are reduced to lower magnitude levels.
To further clarify the benefits of applying emphasis to signals at high frequencies, FIG. 4 illustrates a setup for analyzing the ill-effects of high frequency differential signal transmissions on real, i.e. physical, transmission lines. Transmission lines 14 and 16 are symbolically represented by boxes assigned attributes consistent with the electrical characteristics of a physical transmission line, such as assigning it a resistive value of 50Ω and any appropriate reactive values, if desired. Similarly, a ground plane 12 is represented a pair of boxes assigned appropriate electrical attributes. A signal driver (not shown) applies differential signals via transmitting leads 14a and 16a at the left side of transmission lines 14 and 16, respectively. The applied differential signals travel the length of transmission lines 14 and 16 until reaching receiving leads 14b and 16b and being applied to a receiver (not shown) at the right side of transmission lines 14 and 16. As is customary, 50Ω terminating resistors 18 and 20 couple receiving leads 14b and 16b to ground to reduce signal reflection and maximize the signal-to-noise ratio. In the present discussion, an ideal differential pulse waveform applied to transmitting leads 14a/16a and observed at receiving leads 14b/16b will be compared with an emphasis-shaped waveform also applied to transmitting leads 14a/16a and likewise observed at receiving leads 14b/16b. 
In FIG. 5, examples of an ideal differential pulse signal and an emphasis-shaped differential signal for application to transmitting lead 14a/16a are given different DC voltage offsets for the sake of clarity, so as to avoid overlapping the ideal and emphasis-shaped signals. This DC offset is not critical to the present explanation. The lower set of waveforms, i.e. waveforms 20a and 22a, represent true and complement differential signals from an ideal source, not shown, driven onto transmission lines 14 and 16. The upper set of waveforms, i.e. waves 24a and 26a, represent true and complement differential signals from a driver circuit that shapes the pulses in a controlled manner, i.e. applies emphasis shaping.
An ideal transmission line would only delay a signal by the time it takes the signal to traverse the length of the transmission line, and would not change the shape of the traversing signal. However, this is not the case in a real (i.e. physical) transmission line, particularly when transmitting signals at very high frequencies. In a physical transmission line, a transmitted signal will suffer degradation and have its shape altered as it traverses the transmission line.
With reference to FIG. 6, the lower set of waveforms, 20b and 22b, indicate the shape of the true and complement signals (20a/22a from FIG. 5) issued by the ideal source once they have traversed transmission lines 14/16 and arrived at receiving leads 14b and 16b (FIG. 4). The upper set of waveforms, 24b and 26b, indicate the shape of the true and complement signals 24a and 26a issued by a pulse shaping driver (i.e. with emphasis) after they have traversed transmission lines 14/16 and arrived at receiving leads 14b/16b. As explained above, each set of received complementary signals 20b/22b and 24b/26b would be applied to respective receivers (such as differential amplifiers), which would attempt to recover the transmitted data. However, since both sets of signals 20b/22b and 24b/26b are distorted (i.e. have had their shapes altered as they traversed transmission lines 14 and 16) it is not readily apparent which set of waveforms the receivers would be better able to read and properly recover transmitted data. In other words, it is not clear which set of received waveform signals is of better quality.
One way to discern the quality of received differential signals is to plot an eye diagram of the difference between the true and complement signals in each set of waveforms. FIG. 7 shows two eye diagrams respectively constructed from the two sets of complementary signals 20b/22b and 24b/26b at the receiver end of the transmission line. The construction of an eye diagram is best understood by explaining how it is generally constructed in the field. The base band waveform is typically connected to an oscilloscope whose time base is triggered by the receiver sampler timing once each P seconds. A long sequence of random data is then fed to the transmitter. The result is a supposition of the possible P-second transitions in the waveform, which form a pattern that resembles an eye. As long as the eye is “open”, one can recover the transmitted data, but if the eye is closed, then it is not possible to recover the transmitted data. Thus, the quality of a received signal can be gauged by a determination of how open its resultant eye pattern is.
In FIG. 7, the left diagram is from signals (waveforms 20b/22b in FIG. 6) received from the ideal source (ideal pulse waveforms 20a/22a in FIG. 5), and the right diagram is from signals (waveforms 24b/26b in FIG. 6) received from the wave-shaping driver (emphasis waveforms 24a/26a in FIG. 5). The diagrams cover 3 data bit periods of 400 ps each for a total of 1.2 ns. It can be seen that in the left diagram no eye is visible, i.e. the eye is “closed”, meaning that the ideal square pulses are degraded to such an extent that no discernable data signal can be recovered at the receiving end of the transmission line. The right diagram shows that the emphasis-shaped signals (i.e. the shaped pulses) produce an opened eye 38 in the diagram meaning that the receiver can definitely recover the transmitted signal. The amount of improvement in the received signal is a function of the transmission line and the amount/type of emphasis of the shaped signal.
In the past, circuits for implementing wave-shaping (i.e. emphasis or pre-emphasis) techniques have typically required control logic circuitry having registers and logic comparators to compare a current logic output at the signal driver with a previous logic output in order to identify logic transitions and to determine if emphasis should be applied. Also in the prior art, the output driver itself was typically comprised of two separate, and independent, output driver circuits, one that provided emphasis output voltage levels, and another that provided non-emphasis (i.e. reduced) output voltage levels. The control logic circuitry would select one or the other (or both) of the output drivers depending on whether emphasis should be applied.
For example in FIG. 8, in a prior art pre-emphasis transmitter (only one of a pair of true or complement lines is shown for simplicity), data to be transmitted is applied directly to a non-emphasis signal driver 30, which provides a reduced voltage swing. The pull-up PMOS transistor and pull-down NMOS transistor of non-emphasis driver 30 are made relatively weak and not able to fully pull-up output line 32 to VCC, or to fully pull-down output line 32 to GND. Consequently, this example provides a second signal driver 34 that is activated when pre-emphasis is desired. By having both the first 30 and second 34 signal drivers working in tandem, the output line 32 receives increased current sourcing/draining capability and is thereby driven all the way up to the logic high power rail and all the way down to the logic low power rail.
As shown, this circuit requires that the data signals to be transmitted be applied to a shift register 36 to keep a record of previously transmitted logic signals. The contents of shift register 36 are applied to a digital comparator 38 to identify logic transitions between previous and current output data signals, and the output of the digital comparator 38 is applied to a pre-emphasis controller 40, which also receives the current data to be transmitted and selectively activates second signal driver 34, as needed.
The use of a shift register and digital comparator complicate and increase the overall structure of the output driver. Furthermore, the pre-emphasis circuit of FIG. 8 is a CMOS based circuit, but CMOS circuitry is often not suitable for very high frequency applications. At very high frequencies, one typically requires current based circuitry, such as current mode logic (CML) circuitry.